I'm currently writing a plugin that displays some images - possibly with captions. So, I was wondering whether to use XHTML (<div>s and <span>s) or HTML 5 (<figure> and <figurecaption>) for output.

I image that the answer to this question depends somewhat on the theme being used. Is there a way to figure out which output format the user's theme is using? Or what's the best approach here? What's Wordpress doing to solve this issue?